The present invention relates to apparatus for emitting light.
Lamps using energisation of UV emitting plasmas, particularly for specific spectral wavelength requirements such as curing of chemical compounds and sterilisation purpose are known. They have several advantages over arc lamps such as long lamp life, stable lamp output, and a wide variation of possible lamp envelope designs. Furthermore such lamps are electrodeless and thus the effect of fill materials on electrode material does not need to be taken into account.
Several methods of energising UV light emitting plasmas exist. U.S. Pat. No. 1,482,950 and U.S. Pat. No. 4,042,850 describe non-resonant microwave cavities containing a microwave energised plasma bulb, where one wall of the cavity is constructed of a mesh material which contains microwave energy in the cavity and allows the passage of UV emitted light Since, a UV reflector forms part of the cavity, a compromise has to be reached between optimising the microwave cavity dimensions and optimising the UV light output characteristics.
U.S. Pat. No. 5,166,528 and WO 96 09842A both describe resonant microwave cavities into which are placed electrodeless UV light emitting bulbs for sterilisation purposes. Sterilisation done in this way must be a batch process and articles to be sterilised must not be affected by or substantially absorb microwave radiation since in use, the articles am exposed to the energising microwave field.
WO 97/35624 describes a vessel, to be placed in a microwave field constructed of materials which emit UV light when excited by microwave radiation and which attenuate microwave radiation so as to protect the contents of the vessel from said radiation. Operation is possible within a resonant or non-resonant microwave field but the techniques is suitable only for batch processes and involves complex and costly techniques for the construction of the UV emitting vessel.
GB 2048589A, GB 204225A and GB 2307097A all refer to the energisation of a UV light emitting plasma bulb by microwave radiation coupled to tie bulb via a coaxial system. In each case, provision needs to be made to prevent leakage of microwave radiation and the techniques r ed to do this limit the emission of UV light and the accessibility to the UV emitting plasma bulb.
Thus various methods of microwave energisation for the production of UV light have been developed for chemical and sterilisation processes. In all cases, the methods used for limiting or preventing microwave leakage compromise the UV light emissions by shadowing, In some cases, these method result in the need for a batch process.
Additionally, exposure of the product to be treated, by UV light to microwave radiation is undesirable in some cases which prevents the use of all the above methods expect that of WO97/35624. Even using the techniques of WO97/35624, the product to be exposed has to be small enough to fit into a sterilisation vessel within a microwave cavity and the process must be a batch process.
Apparatus for radiating energy at one or more predetermined wavelength comprising, a housing, a source of microwave energy coupled to and located outside the housing and a window forming part of the wall of the housing, the window being formed form a material which is substantially transparent to radiation at the or each predetermined wavelength and at the wavelength of the microwave source, the window including gas of a predetermined composition at a predetermined pressure contained in a gas-tight enclosure defined by the window material, the gas composition being chosen to emit energy at the or each predetermined wavelength in response to microwave energy from the housing impinging generally on an inner surface of the window, the window being arranged substantially to be opaque at the wavelength of the microwave energy and being arranged to provide an unobstructed radiating path from its outer surface for the energy of the or each predetermined wavelength.
The cavity may be resonant or non-resonant.
The present invention allows products of any size to be irradiated on a continuous process basis by UV or visible light excited by rf (typically microwave) radiation without that light, being reduced by the necessity of providing a method of reducing or preventing rf leakage. In addition, the present invention substantially prevents a product to be irradiated from being exposed to the rf radiation.
Preferably the apparatus includes an electrically conductive rod extending generally from the rf coupling into the cavity. More particularly, the rod will be electrically coupled to the centre conductor of the coupling. The effect of this is more evenly to distribute the intensity of the light across the window.
Preferably at least part of the cavity wall is constructed and arranged as a reflector to direct light which is emitted from the window into the cavity back out of the cavity through the window. This increases efficiency of light emission by using light which is emitted away from the product to be irradiated (i.e. into the cavity) back in the direction of the product.
Additionally or alternatively, one or more separate reflectors may be mounted in the cavity which are constructed and arranged to direct light emitted from the window into the cavity back out of the window. The materials should be transparent to the predetermined wavelength rf energy but reflective to the one or more predetermined light wavelengths. A suitable material is a PTPE-based material.
To further improve efficiency, either of the above reflectors or both may be focusing reflectors.
The window may form a recess into the cavity in which case a product to be irradiated may be inserted into the cavity. Alternatively, the window may be generally planar.
By arranging for the window to be generally tubular and to interconnect two faces of the cavity, it is possible to allow a continuous product to be passed through the tubular window and be irradiated by the one or more predetermined wavelengths of light during its passage therethrough.
Preferably, the window forms an outwardly curving wall of the cavity. This gives a greater spread of light.